Antigen-specific, major histocompatibility restricted recognition by classical T cells is mediated by a T cell receptor (TCR) consisting of a disulfide-linked alpha beta heterodimer. Recently, a second heterodimeric T cell receptor, termed TCR gamma delta, was identified. TCY gamma delta appears to be expressed by a separate T cell lineage distinct from TCR alpha beta-bearing cells. Despite the rapidly accumulating information based on the biochemical nature of the gamma delta receptor and the structure of the gamma and delta genes, there is at present no firm understanding of the function of T cells which bear gamma delta receptors. The goal of this project is to gain insight into the role of TCR gamma delta+ T cells in the immune system. The first step in determining the function of the new lymphocyte population is the identification of an immune response in which that population participates. We have recently found that TCR gamma delta+ cells proliferate in vivo during the primary immune response to certain foreign antigens. For example, the primary immune response to heat-killed mycobacterium tuberculosis is characterized by a 30-fold expansion in TCR gamma delta cells. Evidence that this expansion is associated with specific activation comes from the finding that greater than 30% of TCR gamma delta+ cells in the MT-primed lymph node express high densities of functional IL-2 receptors. Using this primary immune response as a model system, this proposal describes experimental approaches aimed at addressing the following specific questions: 1) what specific ligand(s) are recognized by the population of activated TCR gamma delta cells? 2) which specific gene segments of the gamma and delta loci are utilized in the response to foreign antigen? 3) what is the physiologic effector or regulatory role of TCR gamma delta+ cells in the antigen-specific immune response? The analyses of ligand recognition in the TCR gamma delta repertoire will involve the in vitro cloning of TCR gamma delta-bearing T cells followed by specific antigen activation studies and sequence analysis of the gamma and delta gene segments utilized in these clones. Evaluation of the effector functions of these TCR gamma delta T cells will involve an analysis of lymphokine production both in vitro and in vivo as well as a series of adoptive transfer experiments utilizing purified lymphocyte populations. Finally, attempts will be made to raise specific monoclonal antibodies which recognize gamma delta receptors that are expressed by clones grown out of the MT primed lymph node. By comparing the normal immune response with the immune response generated during in vivo treatment with these anti-gamma delta antibodies, it should be possible to gain yet further understanding of the role of TCR gamma delta+ T cells.